When did X-ray astronomy progress from quick-look sounding rockets to orbiting observatories?

The step change was in December 1970 with the launch of Uhuru, the first real cosmic X-ray satellite observatory. It immediately transformed the subject. In fact, both of my rockets zoomed off after the launch of Uhuru, one only a month after, so I felt there was fierce competition: I was trying to get a good result from 10 minutes of data, and they already had a month of data. However, they had so many exciting findings anyway that I was able to do my own thing as well.

Was the impact of Uhuru such that X-ray astronomers could then say for the first time: "We now have a real sub-discipline with plenty of data and intellectual puzzles"?

Before Uhuru there were exciting problems. X-ray astronomy was taking off as a subject, but Uhuru was a landmark in terms of transforming X-ray astronomy into a true science.

Your contribution to X-ray astronomy is such that you are one of the most highly cited practitioners in space science, judged on publications since 1992. In the last 10 years you’ve been an author of more than 230 papers, which is more than almost any other space scientist during that time. This is a prolific output. What is your philosophy and research style?

Although as a graduate student I did work on instrumentation and rockets, I realized rather quickly that that style of research was not what I wanted to do: I did not want to compete with the many instrument groups that were around at the time. So I shifted into doing a combination of theory and observation, interpreting the results from satellite observatories launched by others. Perhaps my work has been successful because I’ve been involved in most of the satellites, often early on. In this kind of observational astronomy the instruments on each new space observatory must be at least an order of magnitude better than anything before. My experience has been that by getting involved in a new mission at the earliest stage, you’re in a good position to profit from the early discoveries. As the first theorist to see some of the data, you’re able to make good models from thinking hard about what is going on. Also, in some areas I have tended to make models and predictions before launch, so that we can test them when they are launched. I have been much more focussed than some of my colleagues in terms of what we might see. I have known the potential of many of these satellite observatories, in terms of what we might see. What I have done over the years, and this took 15 to 20 years, is to build up the X-ray research group I’ve got now. The group has several postdocs with funding from different sources such as the U.K.’s Particle Physics and Astronomy Research Council and the Royal Society. Our position with the University of Cambridge means that we have a constant stream of excellent research students. These graduate students have been crucial to the success of the group, and I work with them on both theory and observation. That’s been the secret of success in the last decade.

To what extent are you involved in international collaborations?

A large fraction of what I do is in collaboration with others, and I interact with people elsewhere in the U.K., within the Institute of Astronomy and elsewhere in this university, with scientists in many centers in the U.S., as well as Japan and Germany. I am a member of various consortia and so forth, but I don’t particularly like working in large teams. I prefer small collaborations to a gang of 20 to 30 professionals.

Your highly cited research papers kick off with two contributions on  X-ray reflections from cold matter in the nuclei of active galaxies. What’s that all about?

These two papers look at how hard X-rays are produced in active galaxies with black holes accreting matter. The harder X-rays are produced in the accretion disk, and they reflect off cooler material falling into the black hole. By irradiating this matter they trigger certain spectral signatures, such as photoelectric absorption, electron scattering, Compton scattering, and,  in particular, fluorescence. The latter process leads to the production of fluorescent lines in the X-ray spectrum, and the key one is the iron fluorescent line, whose prominence is a combination of fluorescent yield and the cosmic abundance of iron. The line comes from close to the black hole. These two papers are connected with discovering that particular feature, and in computing and predicting all its properties. Basically we established a technique of using the iron fluorescent line as a diagnostic for black hole physics in active galactic nuclei. We predicted how the spectral line would appear in the strong gravitational field around a black hole. It has a skewed broad profile which we predicted, went out and sought, and subsequently found in some active galaxies. It’s a way of probing material in a strong gravitational field, and hereby we think that we have found strong evidence for the huge gravitational field associated with a black hole.

You’ve also had a big hit with your work on the motion of gas within clusters of galaxies, the so-called cooling flows.

Galaxies are found in self-organized groupings called clusters. Although the universe as a whole is expanding, a cluster of galaxies is tied together by the mutual gravitational forces of its members. Within the cluster, the motions of individual galaxies and the intracluster medium constitute a dynamic scenario. In the centers of these clusters the gravitational potential well is large. The intergalactic medium is ordinarily very difficult to detect. But in the centers of clusters the gravitational forces squeeze and concentrate the intergalactic medium so that it becomes visible in X-rays. In radiating to us the gas is losing energy, which is a cooling process. In many clusters the radiative cooling time is less than the age of the universe, and in the middle of some clusters it is below 10 million years, so the issue is, what’s going on there? What’s stopping the gas from getting too cold to see in X-rays? Is the gas continuing to cool or is something stopping it from cooling? The evidence we had back then was that the cooling time was short, and there was evidence of cooler components. The subject is of hot current interest because it has turned into a big puzzle: what is seen at the moment with Chandra and XMM-Newton orbiting observatories is that the gas in these regions does have a short cooling time, and can be cooler by a factor of three than the surrounding gas. But we don’t know why this is so. There are a lot of speculations but we’re currently short on facts. It’s turned out to be a really fascinating area; it is connected with galaxy formation and also with wider aspects of what is going on in clusters and the cooling of the gas.

You’ve been particularly interested recently in the galaxy MCG-6-30-15, and your work on that has attracted attention.

This is the galaxy for which we have the best evidence for the broad skewed iron line, which has a very clear reflection signature. We discovered this broad feature and I’ve worked on it many times since. I am currently working on XMM data from this object. The spectral line is so broad, and skewed to longer wavelengths that we think we are looking to within two gravitational radii of the central black hole in this galaxy. This is therefore one of the key objects to work on in terms of understanding how a central black hole, accreting matter, affects a galaxy. In this case the luminosity of the black hole is 10³⁶ watts and it varies on a time scale of 100 seconds! This is telling us it’s something quite exceptional: 10<^>10 solar luminosities switching on and off in minutes. How does it do that? We think that the only way you can get such profound changes in radiation from such a small object is via accretion onto a supermassive black hole.

What’s the most exciting topic you’ve worked on?

<B>Fabian:I think it must be the black hole work. But I do tend to be the kind of scientist who does one thing on Monday and a totally different thing on Thursday. Normally I have many things I am doing at the same time. I’m lucky that I really am able to multiplex my time. So my answer to your question could well change on a daily basis!

Which forthcoming space science missions are you active with?

I am involved with Astro-E2 which is a mission to replace Astro-E (a Japanese mission), and this should go up in 2005 and is going to have very fine X-ray spectral resolution. I am involved with Constellation X which is the next big NASA X-ray satellite. Then there’s a European mission, XEUS, which is a very powerful X-ray telescope.

Finally, what are the main intellectual challenges at this time in X-ray astrophysics?

There are a lot of issues to do with how radiation is released by matter falling down a black hole. Can we test issues to do with strong field gravitation, for example? We’re only just starting to confront these matters. For myself there are a lot of interesting issues to do with the role of gas in galaxy formation. In some sense at the moment cosmology is starting to be "solved."  It would be great if X-ray astronomy could contribute significantly to dark energy studies. At the moment it’s not clear it can. A lot of astrophysicists are looking at how gas cooled to form proto-galaxies. What role does the central black hole have to play in galaxy evolution? These are issues where X-ray astronomy has much to offer.